Methods of and means for controlling electromagnets



Jan. 20, 1959 H. E. HODGSON 2,370,382

METHODS OF AND MEANS FOR CONTROLLING ELECTROMAGNETS Filed June 25, 1954 L1 La United States Patent ice METHODS OF AND MEANS FOR CONTROLLING ELECTROMAGNETS Howard E. Hodgson, Wauwatosa, Wis., assignor to Cutler- Hammer, Inc, Milwaukee, Wis., a corporation of Delaware Application June 23, 1954, Serial No. 438,815

3 Claims. (Cl. 317-123) This invention relates to improvements in methods of and means for controlling electromagnets. It relates more particularly to improved methods of and controllers for demagnetizing large lifting magnets.

To avoid the undesirable and sometimes dangerous effect of permitting a load to dribble from an electromagnet when the load is to be released, it is necessary to de-magnetize the magnet in a very short time. This is especially true in connection with lifting magnets, since it is often desired that loads be dropped while the lifting magnet is swinging or otherwise moving. Here a reduction is the dribble time of only a fraction of a second is very important. 2

It is common practice to de-magnetize magnets by reversing the power connections to the magnet and connecting a resistor in series with the magnet and its power source. The magnet current decreases exponentially at a rate determined by the value of the resistor and theinductance of the magnet, and it becomes zero when the magnet voltage drop is equal to the voltage of the supply source. Thereafter the current reverses its direction of flow and produces reversed magnetism to overcome residual magnetism in the magnet iron. Even when the ohmic value of the series resistor is such as to result in a most rapid decay of current and increase of reverse current that is permissible without damaging the magnet insulation or the resistor, the rate, near the end of the demagnetizing period at which the residual magnetism in the magnet is overcome, is relatively slow since reversed current increases exponentially. Thus the residual magnetism is overcome relatively slowly and dribbling of certain types of loads results.

Applicant has discovered that it is not essential, in order to properly drop the load, to maintain the reverse power connections to the magnet until all residual magnetism .is overcome. The load may be cleanly dropped in shorter time by taking advantage of the fact that residual magnetism is not evenly distributed over the face of magnets. This advantage may be realized by disconnecting the magnet from the supply source before residual magnetism is entirely overcome and while reversed magnet current is increasing at a relatively rapid rate, thus to alter rapidly the distribution of magnetism over the magnet face.

In practice it has been found that where the time constant of the magnet discharge circuit is fixed, the voltage of the magnet is a reliable and sufiicient measure of the magnetic condition of the magnet to permit its being used to determine when the magnet is to be disconnected from the power source and this invention takes advantage of that fact.

The primary object of the invention is to provide an improved method of and means for controlling and de-magnetizing electromagnets.

' Another object is to provide a magnet controller affording a shorter and more uniform dropping time than p vious controllers have provided.

Other objects and advantages of the invention will hereinafter appear.

, 2,870,382 Patented Jan. 20, 1959 The accompanying drawing illustrates diagrammatically a control circuit embodying the invention which will now be described, it being understood that various modifications may be made in the embodiment illustrated and that other embodiments of the invention are possible without departing from the scope of the appended claims.

In the drawing, lines L1 and L2 representa source of electrical power for energization of a system which comprises: an electromagnet 10 having terminals X and Y; a contactor 12 having an operating winding 14, normally open contacts 16 and 18 and normally closed contacts 20 and 22; a contactor 24 having an operating winding 26, normally open contacts 28 and 30 and normally closed contacts 32; a LIFTDROP switch 34 and resistors 36, 38, 40 and 42.

The energizing circuit for winding 26 is arranged so that current flow through said winding is insufiicient to lift contactor 24. When switch 34 is open, current flows from line L1, through resistors 36 and 38, through winding 26, and thence through resistor 40 to line L2. When closed, switch 34 short circuits resistors 36 and 38.

Contactor 24 is mechanically connected, as at 44, so that it is lifted by contactor 12, when the latter is actuated. Once so lifted, contactor 24 is maintained in lifted posiion by current flow through winding 26 except as hereinafter described.

A working cycle of the magnet is initiated by closing switch 34 to the Lift position. Winding 14 of contactor 12 is energized through switch 34 and contacts 16 and 18 close and contacts 24) and 22 open. The magnet 10 is energized by current flow from line L1, through contacts 18, through magnet 10 from terminal X to terminal Y and through contacts 16 to line L2.

Contactor 12 operates contactor 24 to close contacts 28 and 39 and to open contacts 32. The actuation of these contacts has no effect on the circuit, under normal conditions, while switch 34 is closed.

When switch 34 is opened to Drop position, winding 14 is de-energized and contacts 16 and 18 open. Contacts 20 and 22 close to reverse the connection of magnet 10 to lines L1 and L2. This circuit is traced from line L1 through resistor 36, contacts 30, contacts 20, magnet 10 from terminal Y to terminal X, contacts 22 and 28 and thence through resistor 40 to line L2. The inclusion of resistors 36 and 40 in the reversed power connection reduces the ratio of inductance to resistance in the circuit and increases the rate at which the energy stored in magnet 10 is returned to the supply source Ll-LZ.

When the voltage drop across the magnet is reduced to the voltage across lines L1 and L2, current flow in the magnet is reduced to zero. Thereafter the direction of current flow reverses and begins to increase in the reverse direction. The magnet is magnetized in a direction of polarity to overcome its residual magnetism.

Contactor 24 is held lifted by its winding 26 after winding 14 of contactor 12 is de-energized to open contacts 16 and 18 and close contacts 20 and 22. Winding 26 of contactor 24 is thereby connected in in series circuit with resistor 38 across the terminals of magnet 10. Thus the voltage drop across magnet 10 is applied to resistor 38 and winding 26. The voltage drop which appears across winding 26 is proportional to, but is less than, the voltage drop across the magnet 10. When the magnet voltage is reduced to a selected value, the voltage across winding 26 is reduced to a value below that effective to maintain contactor 24 lifted. Then contacts 28 and 30 open to interrupt the power connection to the magnet abruptly. Contacts 32 close to complete a loop circuit including contacts 20 and 30, magnet 10 and a resistor 42. All of the energy remaining in magnet 10 is dissipated in resistor 42.

When, as in the case of a lifting magnet, the safety of persons and equipment may depend upon release of a contactor at a selected voltage, it is desirable to employ a contactor having a very wide operating voltage differential to insure release of the contactor at the selected voltage. Also, for economy as well as for safety, it is desirable that the controller circuit be very simple. For these reasons, it has been the practice in the past in controllers of this class, to measure the magnet voltage in a contactor which was actuated at a voltage as close to the supply line voltage as possible, taking into account possible undervoltage conditions, and which opened at a low magnet voltage thus to provide a wide operating differential. As a result, the reverse current in the magnet was necessarily relatively large before the magnet was disconnected from the power source and residual magnetism was substantially entirely overcome.

' To utilize the change in total magnetism, in dropping the load in accordance with my method, the magnet must be disconnected from the supply while reverse current,

and the resulting magnetism opposing residual magnetism,

is increasing relatively rapidly. The magnet must'be disconnected from the supply when the magnet voltage has a magnitude which is only a little less than the voltage of the supply source.

This is accomplished in the control circuit illustrated in the drawing. Contactor 24 has a wide operating differential. The voltage required to be applied to winding 26 to actuate contactor 24 is higher than the voltage applied to it in either position of switch 34 and is lifted only by operation of contactor 12.

On the other hand, when the magnet is to be de-magnetized the winding 26 is connected in series with resistor 38 across the magnet so that whereas the voltage applied to winding 26 is proportional to magnet voltage, but is substantially less than magnet voltage. The magnet voltage may be only a little below the voltage of the supply source when the magnet is disconnected, but the actual voltage across'winding 26 is low and the operating differential of contactor 24 is great.

Another important advantage of this control circuit is that since winding 26 is not required to lift contactor 24, the resistance of this coil may be made relatively great to limit current therethrough. This results in less heating of winding 26 and less change in its resistance so that said winding 26 will release contactor 24 at the proper voltage with greater accuracy.

I claim:

1. In combination: a source of electrical power; an electromagnet; a first contactor having first and second normally open contacts and first and second normally closed contacts; a second contactor having a normally closed contact and first and second normally open contacts; first and second and third resistors; a switch having a first and second terminal; an energizing circuit for said first contactor comprising circuit means extending from one side of said source through said first contactor to the first terminal of said switch; an energizing circuit for said second contactor comprising circuit means extending from said one side of said source through said second con tactor to said second terminal of said switch; said switch acting in a first position to connect said first and second terminals to the other side of said source and in a second position to disconnect said first and second terminals from said other side of said source and from one anotherya first series circuit extending from said one side of said source to said other side of said source and comprising in order said first normally open contact of said first contactor, the operating winding of said electromagnet, a junction point, said first resistor, said normally closed contact of said second contactor, said first normally open contact of said second contactor and said second resistor; a second series circuit extending from said one side of said source to said other side of said source and comprising in order said second normally open contact of said second contactor, said first normally closed contact of said first contactor, said junction point and said second normally open contact of said first contactor; said second normally closed contact of said first contactor being connected to bridge said operating winding of said electromagnet, said junction point, said first resistor and said normally closed contact of said second contactor; and said third resistor being connected from said second terminal of said switch to a point in said first series circuit intermediate said first normally open contact of said second contactor and said second resistor; said second contactor being unable to close its normally open contacts or open its normally closed contact, but being able to maintain the same so operated; and means to so operate said contacts of said second contactor when said first contactor is energized.

2. In combination, a source of electrical power, an electromagnet, first and second electroresponsive con tactors, means for establishing energizing connections for the first contactor across said source to complete power magnetizing connections for said electromagnet across said source, means normally energizing said second contactor sufficiently to hold it actuated but insufiiciently to actuate it, means affording mechanical actuation of said second contactor as a result of the aforementioned operation of said first contactor, a resistor, and means responsive to drop out or said first contactor whenever said second contactor is actuated to establish reverse demagnetizing' connections for said electromagnet across said source and to connect said resistor in circuit with the operating winding of said second contactor across said electromagnet to subject the latter winding to a voltage proportional to the voltage across said electromagnet. said second contactor responding to drop out and interrupt the last mentioned demagnetizing connections for said electromagnet when the voltage across said electromagnet is reduced to a given value.

3. In combination, a source of electrical power, an electromagnet, a first contactor having an electro-respon sive operating winding and a normally open and a normally closed contact, a second contactor having a normally open contact, means for establishing and interrupting energizing connections for the operating winding of said first contactor across said source, means in circuit with the operating winding of said second contactor across said source continually subjecting the latter winding to a voltage sufficient to maintain its normally open contact closed but insufficient to effect its closure, means affording mechanical actuation of said second contactor to close its contact as a result of operation of said first contactor in response to energization of the latter operating winding, means including the normally open contact of said first contactor for establishing normal power magnetizing connections for said electromagnet across said source, whenever the latter contact is closed, a resistor, and means including said normally closed contact of said first contactor and said normally open contact of said second contactor for establishing reverse power demagnetizing connections for said electromagnet across said source and connecting said resistor in series with the operating winding of said second contactor across said electromagnet to subject the latter winding to a voltage proportional to but less than the voltage across said electromagnet when the last mentioned contacts are both closed, said second contactor being responsive to reopen its normally open contact whenever the voltage across said magnet is reduced to a given value.

References Cited in the file of this patent UNITED STATES PATENTS Leitch Aug. 12, 1952 

